Some electronic file systems include information recording disks for storing files each having information representing documents. As the number of files increases and the mount of information in each file increases, a longer time is spent in the access to a desired file. Only a small portion of the access time is used in writing and reading information into and from an information recording disk. A great portion of the access time is used in selecting a desired disk and moving the selected disk to a disk drive.
Electronic file systems of a multi-disk-drive type have been developed in order to shorten an access time. The electronic file system of the multi-disk-drive type has a plurality of disk drives. In this electronic file system, while a first disk drive is executing information reading and writing processes on a first disk, a second disk can be moved to a second disk drive to shorten an access time. As will be explained later, a prior art electronic file system of the multi-disk-drive type has same problem.
With reference to FIG. 1, a prior art electronic file system of the multi-disk-drive types includes a plurality of work stations 1 (WS-A, WS-B, and WS-C), a file controller 2, a multi-drive controller 3, and a multi-drive section 4. The work stations 1 are also referred to as terminal devices.
Various commands can be inputted via the work stations 1. The file controller 2 connects the work stations 1 and the multi-drive controller 3. The input commands are transmitted to the multi-drive controller 3 via the file controller 2. The multi-drive controller 3 controls a mechanism within the multi-drive section 4, and also controls data (information) reading and writing operation of the multi-drive section 4 in response to the input commands. The multi-drive controller 3 outputs read-out data to the file controller 2. Then, the read-out data is transmitted to the work stations 2, being indicated and processed in the work stations 1.
The multi-drive section 4 includes a plurality of data (information) recording disks 5, a carrier mechanism 6, and two disk drives (decks) 7. The disks 5 are normally contained in a disk rack or a disk holder. The carrier mechanism 6 selects one of the disks 5 and carries the selected disk 5 from the disk rack to one of the disk drives 7 in response to a control signal fed from the multi-drive controller 3. Under conditions where a disk 5 is set in a disk drive 7, the disk drive 7 can read out and write information from and into the disk 5 in response to a control signal fed from the multi-drive controller 3. While one of the disk drives 7 is executing data (information) reading and writing processes on a first disk, a second disk can be moved by the carrier mechanism 6 to the other disk drive 7 to shorten an access time. After an information reading process or an information writing process is completed, the carrier mechanism 6 can return a disk 5 to the disk rack in response to a control signal fed from the multi-drive controller 3.
A description will now be given of operation of the prior art electronic file system of FIG. 1 which occurs under exemplary conditions. With reference to FIG. 2, data (information) reading commands A-1, B-1, and C-1 are sequentially transmitted from the work stations WS-A, WS-B, and WS-C to the file controller 2 respectively. The data reading command A-1 is immediately transferred from the file controller 2 to the multi-drive controller 3, while the data reading commands B-1 and C-1 are held in the filer controller 2. The multi-drive controller 3 controls the multi-drive section 4 in response to the command A-1. Specifically, the multi-drive controller 3 activates the carrier mechanism 6, replacing the current disk 5 by a desired new disk 5 in a first disk drive 7. Then, the multi-drive controller 3 activates the first disk drive 7, reading out data from the desired disk 5 in compliance with the command A-1. In this way, the command A-1 is executed. The read-out data required by the command A-1 is returned from the multi-drive controller 3 to the work station WS-A via the file controller 2 as a response to the command A-1. When the work station WS-A retrieves the response to the command A-1, the work station WS-A transmits a subsequent data reading command A-2 to the file controller 2. Next, the data reading command B-1 is transferred from the file controller 2 to the multi-drive controller 3. The multi-drive controller 3 controls the multi-drive section 4 in response to the command B-1. Specifically, the multi-drive controller 3 activates the carrier mechanism 6, replacing the current disk 5 by a desired new disk 5 in a second disk drive 7. Then, the multi-drive controller 3 activities the second disk drive 7, reading out data from the desired disk 5 in compliance with the command B-1. In this way, the command B-1 is executed. The read-out data required by the command B-1 is returned from the multi-drive controller 3 to the work station WS-B via the file controller 2 as a response to the command B-1. When the work station WS-B receives the response to the command B-1, the work station WS-B transmits a subsequent data reading command B-2 to the file controller 2. Next, the data reading command C-1 is transferred from the file controller 2 to the multi-drive controller 3. The multi-drive controller 3 controls the multi-drive section 4 in response to the command C-1. It is now assumed that data required by the command C-1 is contained in neither the current disk 5 in the first disk drive 7 nor the current disk 5 in the second disk drive 7. Thus, the multi-drive controller 3 activates the carrier mechanism 6, replacing the current disk 5 by a desired new disk 5 in the first disk drive 7. Then, the multi-drive controller 3 activates the first disk drive 7, reading out data from the desired disk 5 in compliance with the command C-1. In this way, the command C-1 is executed. The read-out data required by the command C-1 is returned from the multi-drive controller 3 to the work station WS-C via the file controller 2 as a response to the command C-1. When the work station WS-C receives the response to the command C-1, the work station WS-C transmits a subsequent data reading command C2 to the file controller 2. During a later period, similar processes are sequentially executed for the subsequent data reading commands A-2, B-2, and C-2.
Each of the commands A-1, B-1, C-1, . . . requires reading out an amount of data (information) which corresponds to one page of predetermined-size documents. FIG. 3 is a timing chart showing conditions of the data reading operation related to FIG. 2. In FIG. 3, the abscissa denotes an elapsed time (second), and the ordinate denotes a page number. In FIG. 3, numerals written below the boxes containing the command-denoting characters represent elapsed time sup to the moments of the completion of the data reading processes. As shown in FIG. 3, the commands from the work stations are sequentially executed at equal time pitches of 18 seconds. The disk replacement in the disk drives 7 require such long pitches.
In usual cases, a group of the commands A-1, A-2, . . . from the work station WS-A, a group of the commands B-1, B-2, . . . from the work station WS-B, and a group of the commands C-1, C-2, . . . from the work station WS-C relate to different disks 5 respectively. Since the number of the disk drives 7 is two, only two of the disks 5 can be simultaneously exposed to the data reading process. Each disk replacement in the disk drives 7 spends a certain time. The disk replacement includes the transfer of a disk 5 between the disk rack and a disk drive 7 by the carrier mechanism 6. Accordingly, in the case where commands from the work stations 1 are sequentially executed as mentioned previously, when the frequency of access to different disks 5 increases, the execution of the commands tends to be interrupted by the disk replacement and the transfer of the disks 5.